The present invention relates to a combination of a mixing device and mold for the production of moldings from flowable reactive components. The assembly has a supply channel with changes in cross-section extending between the mixing chamber of the mixer and the mold cavity and the mold parting plane runs along the supply channel. Such combinations of mixer and mold are particularly useful for the foaming in situ of polyurethane reaction mixtures to form moldings.
Such combinations can be designed in different ways. The mixer can be integrated into the mold. It can be fixed to the mold or it can be put on the mold only for the filling process.
With such a combination, either the mixer should operate in a self-cleaning manner, or the mixture residue remaining in the mixing chamber can be released with the molding at the same time from the mold. Mixers which have to be rinsed were once used for such purposes, but are no longer used today due to the technical problems and disadvantages connected with cleaning.
In German Offenlegungsschrift No. 26 12 812 (corresponding to U.S. Pat. No. 4,141,470) a throttling device is movable in a bore extending transversally to the guide bore of a discharge piston in a position throttling the cross-section of the guide bore. The throttle device has a through-hole, the cross-section of which corresponds to at least the cross-section of the discharge piston. It is possible, by means of the throttle device, to influence the mixing chamber pressure and the outlet speed of the reaction mixture from the mixing chamber. It is also possible to achieve after-mixing and to bring about a relaxation in the flow in the connecting outlet pipe. This mixer fails, however, when used for the production of highly reactive reaction mixtures which solidify within a few seconds, particularly when large mold cavities must be filled. When the mixing process ends, the throttle device must first be brought out of its operative position into the inoperative position, before the mixing chamber and the connecting outlet pipe can be emptied by means of the discharge piston. Due to the inertia of the control processes and the clearance necessarily contained in the control system, the movement of the discharge piston cannot be introduced until the throttle device has safely assumed its inoperative position. Since the movement of the throttle device and subsequent disengaging of the discharge position takes about one second, the mixture introduced into the mold cavity and present in the outlet tube or in the mixing chamber would already be partially hardened before the start of the cleaning process. Problems thereby naturally arise.
On the other hand, it is known from German Offenlegungsschrift No. 32 16 647 (corresponding to European Pat. No. 0,093,356), to provide in the supply channel, a substantial reduction of the cross-section in relation to the mixing chamber and to provide multiple deviations and changes in cross-section, in order to achieve an after-mixing of the reaction mixture by the changes in the flow relations thus caused. With this mixer the cleaning process can be started more quickly since no moveable throttle device is present at the outlet of the mixing chamber. The changes in cross-section and path deviations provided in the supply channel are however formed such that they simply cause after-mixing. A relaxation section is not present. Due to the substantial deviations, the reaction mixture blasts into the mold cavity at a high speed. The inclusion of air thus cannot be avoided. The separation of the flow and spraying connected therewith can cause surface contours within the molding, which contours damage the homogeneity of the molding and reduce the mechanical strength.
Similar after-mixers are described in U.S. Pat. No. 3,924,989.
An object of the invention is to be able to influence the mixing chamber pressure and the outlet speed of the reaction mixture from the mixing chamber, as to enable an after-mixing, whereby highly reactive systems, such as those which harden within a few seconds, are also processible. At the same time. particular attention should be paid to the relaxation of the flow, so that during the form filling process, a continuous flow front is ensured and the total quantity of mixture required for the production of the molding can be introduced into the mold before the hardening has passed a critical point.